For safety purposes, an increasing number of motor vehicles possess detection systems including electronic boxes mounted on each of the wheels of the vehicle, enclosing sensors dedicated to measuring parameters such as the radial acceleration of the wheel, and the pressure and the temperature of the tire fitted to this wheel.
These monitoring systems are conventionally equipped with, on the one hand, electronic boxes 1 (also called ‘wheel units’) mounted on each of the wheels of the vehicle and incorporating, in addition to the aforementioned sensors, a microprocessor 10, a memory and a radiofrequency transmitter, and, on the other hand, with a central unit (not depicted, mounted on the vehicle) for receiving the signals transmitted by the radiofrequency transmitters of each wheel, including an electronic control unit (or ECU) incorporating a radiofrequency receiver connected to an antenna.
These wheel units periodically provide the central unit with measurements of basic parameters pertaining to each tire-pressure, temperature, acceleration. These measurements can then be exploited in target applications, notably:                automatic learning by the sensors or the monitoring of the position of the sensors themselves;        location of the wheels in order to monitor the tire pressure corresponding to each wheel located,        detection of tire overload and tire wear.        
The measurements, performed by dedicated sensors, are converted and transmitted to a microprocessor incorporated into the central unit via transmitter/receiver devices equipped with antennas and with a CAN bus. The collection of means implemented (sensors, central unit, communications network via the transmitters/receivers) forms a tire pressure monitoring system known conventionally by the English-language system abbreviation TPMS.
The numerical data supplied by the wheel units to the central unit indicate successive levels of variation. From these values, the central processing unit can, after filtering and sampling, formulate pressure, temperature, wear and/or overload signals for the purpose of transmitting these to the vehicle onboard computer.
One important aspect of the use of TPMS systems relates to reducing the energy consumption. Specifically, these systems, which use wireless communications means for long periods of time, are powered by energy sources of limited capacity, for example battery cells or inductive generators. Power consumption reducing circuits are known.
In general, in order to minimize the energy consumption of these TPMS systems, it is known practice to detect two chief modes of operation, namely a so-called “running” mode during which the vehicle is in motion, and a so-called “non-running” mode during which the vehicle is stationary, and to implement methods for automatically switching the wheel units of these TPMS systems to standby or “sleep” mode during the “non-running” modes of operation, the wheel units then not communicating with the electronic control unit.
These methods thus propose concentrating the pressure monitoring and the associated communications into the phases during which the vehicle is running. However, in order to detect these so-called “running” and “non-running” modes of operation, it is necessary to have a high level of responsiveness to detection of the onset of motion of the vehicle (for example, to respond in 16 seconds or less). This then entails periodically (at least every 16 seconds) measuring the radial acceleration of the vehicle to make it possible to detect that the latter has begun to move, and doing so throughout the duration of the mode of operation referred to as “non-running”.
Thus, although such methods allow enough of an energy saving for the “running” modes, they do not, however, offer satisfactory efficiency because a large majority of the energy consumption of the wheel units of the TPMS systems takes place during the “non-running” phases, which represents 95% of the life of a motor vehicle.
In addition, TPMS development is chiefly targeted toward two objectives, namely:                reducing the size of the power sources (and, therefore, the capacity thereof), and        implementing new functions, leading to additional power consumption.        
The known devices and methods are unable to meet these requirements without significantly affecting the life of the wheel unit of the TPMS system. At the present time, the life of a wheel unit of a TPMS system is of the order of around ten years.
Another disadvantage with the known devices and methods is that they do not propose any sufficiently satisfactory solution regarding the responsiveness of the system to detecting that the vehicle has begun to move.